The present invention relates generally to liquid crystals, and more particularly to ferroelectric liquid crystals.
Ferroelectric liquid crystals (FLCs) have the potential to displace their nematic counterparts, which currently dominate today's $83 billion dollar per year display market (1). Advantages of FLCs include much faster switching speeds, dramatically lower power consumption and superior resolution relative to nematic LCDs.
The spontaneous molecular polarization of FLCs, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. In this confinement, a low electric field of only a few volts can switch the FLC between two equally stable states with opposing polarization directions (FIG. 2b). This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the “ON” state. The power required to run FLC displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, FLCs can switch hundreds of times faster than a nematic display, which can only be driven actively in one direction (and passively relax into their “OFF” state over tens of milliseconds).
FLCs also show high resolution due to the very thin gaps allowed by their mode of action. In the transmission mode, they are typically less than 2 μm thick, compared to 4 to 8 μm in nematic LCDs. In reflective FLC displays, this thickness is reduced by half, resulting in a thickness of less than 1 μm. Thus, pixels as small as 5 μm have been demonstrated. Another advantage of FLCs is the 180 degree viewing angle (available because the molecules switch in the same plane as the screen, not in and out of that plane, like nematic displays).
Most of the difficulties in manufacturing FLC displays can be traced to control of their nanostructure inside the devices. The molecules spontaneously organize into layers (a smectic phase). That is not enough to make them ferroelectric, however. The long axis of the molecules must have a preferred tilt relative to the layer normal (FIG. 2c): if the long axis of the molecules simply points along the layer normal, there is no spontaneous dipole in the plane of the layers. Tilt alone is still not enough to make a ferroelectric: they must also be chiral, so that the molecules are capable of discriminating between two different tilted states that have opposite direction of the dipole in the plane of the layers. But having a dipole component in the layer is again not enough to give a macroscopically ferroelectric material: forcing adjacent layers to point their dipoles in the same direction costs free energy, so the system tends to adopt a “spiral staircase” arrangement of the dipoles, resulting in no macroscopic dipole. To persuade the layers to conform to a single, uniform orientation of their dipoles requires the confinement to a thin gap, so that surface effects are as important as bulk effects. By tailoring the surfaces so that the molecules have a strong incentive to lie parallel to the walls of the cell, two distinct orientations can be favored over all others (eliminating most of the stairs in the “spiral staircase” so that the dipoles no longer have the option of cancelling each other out). That insight is famous in the world of liquid crystals: the concept of a “surface stabilized ferroelectric liquid crystal” (SSFLC) is due to Noel Clark and Sven Lagerwall in 1980. Over 20 years has elapsed since the seminal discoveries regarding SSFLCs were made, leading to the current renaissance of the field as more and more IP comes off patent. The ideal FLC display layer structure is, thus, the “bookshelf geometry,” where the layers are orthogonal to walls of the cell and the molecules in the layers are, on average, oriented parallel to the walls of the cell (FIG. 2a); the LC director is then forced to align parallel to the surface, in one of two discrete orientations that satisfy its required tilt relative to the layer normal (FIGS. 2b and c). A consequence of perfect bookshelf alignment is that an electric field can be used to “pop” the director orientation into one or the other of the two “allowed” orientations; after the field is removed, the director does not reorient (FIG. 2c), so it is stable when voltage stops being applied. Soon after the discovery of SSFLC, however, it was realized that it is no simple matter to achieve the “bookshelf geometry.”
In practice, FLC layers are often oriented in chevrons, with two types of layers that tilt symmetrically away from the bookshelf orientation (4) (FIG. 3). This is a serious problem for FLC devices, because without bookshelf geometry, it is difficult to obtain bistability. Indeed, the few FLC devices on the market today are not bistable, and use a continuously imposed electric field to hold the molecules in a chosen optic axis state, eliminating one of the great advantages of FLCs—low power consumption (5). Zigzag defects are a visible manifestation of the formation of chevron layer structure: the boundaries of domains with different chevron directions result in “leakage of light”—bright lines when the FLC cell is viewed through crossed polarizers (FIG. 1). Such optical defects render a device useless as a display. In an attempt to solve the foregoing problems, much effort has been expended.
One such approach is to use obliquely deposited amorphous silicon as an alignment layer that forces the layers into bookshelf geometry. Unfortunately, since the silicon must be deposited in a vacuum chamber, this method is expensive and difficult to generalize to many types of cell configurations. Another method involves applying a high voltage, low frequency electric field across a cell that has the undesired “chevron geometry” to induce quasi-bookshelf geometry, but these cells have slower switching speeds than the same cell with chevron geometry. Since the quasi-bookshelf geometry is not thermodynamically induced, the alignment is lost if the cell is heated into another LC phase.
In-situ polymerization in a FLC host is another approach described in the literature (6, 7, 8). In this approach, reactive or ionic species for polymerization are present when the LC is loaded in the cell and a subsequent UV irradiation step is required to polymerize the species (with its concomitant damage of organic materials in the device). In other approaches, the polymer forms a separate phase from the host, reducing the optical uniformity of the LC layer. In addition to the disadvantages mentioned, the previous attempts adversely affected the operating voltage and switching speed. Commercialization of an approach involving in situ polymerization has not been forthcoming despite many years of effort.